How do differents factors affect seed germination?

The essential factors for seed germination are
i) Moisture or water
ii) Oxygen
iii) Temperature
iv) Light
Water A dormant seed contains 10-15% of water and is generally dehydrated.
So the dormant seed has to absorb water to become active and exhibit germination.
Water makes the seed coat soft, causes it to rupture after swelling and start germination.
Water is also needed to bring in the dissolved oxygen for use by the growing embryo.
OxygenIn the dormant condition the seeds respiratory rate is very low and so oxygen is required in very small quantities.
But for germination, oxygen is needed in large quantities.
The seeds obtain this oxygen from the air contained in the soil.
Seeds sown deeply in soil, fail to germinate because of lack of oxygen.
Ploughing aerates the soil and helps in good germination.
TemperatureGermination can take place over a wide range of temperature
(5 - 40o C), but the optimum for most of the crops is between 25o C to 30o C.
LightLight has varied effects on germinating seeds of different plants. Some seeds need light for germination,
while in some seeds germination is hindered by light.